High performance aircraft require systems for thermal management and cooling. In typical modes of operation, fuel, lubricating oil for the engine, and hydraulic fluid used in the various hydraulic systems as well as the aircraft mounted accessory drive (AMAD) may require cooling. Additionally, the avionic systems of the aircraft will require cooling during operation, some by a liquid coolant and others by cool air. Concurrently, at low altitude or on the ground or at other relatively high temperature operating environments, the aircraft cabin requires cooling while at relatively low temperature altitudes such as at cruise altitude for a jet aircraft, the cabin will require warming. Through all of this it is generally desirable to heat the fuel delivered to the main propulsion engines to maximize the efficiency of the engines.
The low quality waste heat from avionics, air conditioning, etc. in aircraft is customarily transferred to fuel in a tank and then ejected from the aircraft via burning the fuel. This approach works well when the aircraft is burning fuel at a rate sufficient to keep the fuel tank temperature within design bounds. When the fuel tank temperature rises to a maximum design point, however, operations must be modified to allow the fuel to cool. Thus, high performance aircraft may have thermal limits equally confining as the more familiar operational limits such as the V-G envelope, service ceiling, fatigue limits, etc.